In a conventional wire-bound telephone network each subscriber is provided with a pair of copper wires for communication. The copper wires connect the subscriber point-to-point to a local exchange or switch and then to another subscriber. These copper wires are expensive to install and maintain. Furthermore, each pair of copper wires typically has only enough bandwidth to handle one conventional unmodulated call at a time.
Attempts have been made to enable a subscriber terminal to handle more than one call at a time across a wire-bound network. One known approach is the "call waiting" procedure. According to this procedure, in a wire-bound Public Switched Telephone Network (PSTN), when an incoming call finds a telephone busy with an ongoing call, a "door knocking signal" informs the communicating parties in the ongoing call that an incoming call is waiting. Because of the limited bandwidth of the copper wires, the called party must park the ongoing call to switch to the incoming call. The called party may then switch back to the ongoing call. A problem with this approach is that in many cases the ongoing call is important, and the called party may be unwilling to park the ongoing call, thereby making access by the incoming call impossible.
There is also a problem associated with making an outgoing call from a telephone in a wire-bound network if the telephone line is already being used by another telephone. If a telephone line is in use it is not possible to start a new call without interrupting or terminating the ongoing call.
A digital (so-called "2B+D") interface has been developed which allows two calls to be conducted simultaneously over a single pair of wires using 2 times 64 or 128 Kbits/second for the actual communication and 16 Kbits/second for signalling. This interface is limited to only allowing two calls to be conducted simultaneously.
As an alternative to copper wires, Radio in the Local Loop (RLL) systems have been designed to replicate the functions of the wire-bound subscriber network. Typically, these types of systems employ radio terminals that are fixedly installed to allow communication to and from subscribers. The radio terminals communicate with radio base stations over a radio air-interface, the radio base stations being interfaced with the PSTN. In this way, the radio air-interface takes the place of the copper wires in the wire-bound network, over a portion of the communication pathway. As will be appreciated, RLL systems have particular applicability in areas where it is difficult (or not cost efficient) to run wiring, e.g., in rural areas or inside office buildings. The radio terminals used in RLL systems can be similar to the mobile radio terminals used in cellular radio telephone systems.
A problem with RLL systems is that the RLL terminals, like the wire-bound terminals, can handle only one call at a time. This is partly due to the nature of traffic channels and control channels in radio systems. In radio systems, traffic channels carry speech or user data between a radio terminal and a radio base station, whereas control channels are used for setting up traffic channels and do not carry traffic. There are several different types of control channels, such as paging channels which are used for signalling downlink from the radio base station, and access channels which are used for signalling uplink towards the radio base station. Control channels between a radio base station and a radio terminal are used by a radio terminal when the terminal is not communicating on a traffic channel, for example, when first setting up a call. Therefore, where there is a call going on via a traffic channel between a radio terminal and a radio base station, the radio terminal cannot set up a second independent call using a control channel.
Thus, while RLL terminals permit a subscriber to include a third party in an ongoing call or park an ongoing call to contact a third party, these terminals are limited to transmitting or receiving one call at a time. If a subscriber wants to be able to transmit or receive more than one call at a time, another subscription must be obtained and another terminal must be installed, doubling the cost as compared to a single subscription.